For many years now, telecommunications carriers have been deploying packet-switched networks in place of, or overlaid upon, circuit-switched networks for reasons of efficiency and economy. Packet-switched networks such as Internet Protocol (IP) or Ethernet networks are intrinsically connectionless in nature and, as a result, suffer from Quality of Service (QoS) problems. Customers value services which are guaranteed in terms of bandwidth and QoS.
It is desired to use Ethernet switches in carriers' networks. Use of Ethernet switches in carriers' networks would have the advantages of interoperability (mappings between Ethernet and other frame/packet/cell data structures such as IP, Frame Relay and ATM are well known) and economy (Ethernet switches are relatively inexpensive compared to IP routers, for example). It would also provide a distinct advantage of being the principal technology used by enterprises that require a wide area network service from a carrier and therefore able to work in a native mode. In view of the above, there have proposals to implement ‘Connection-oriented Ethernet’ (CoE) networks where managed traffic paths (connections) are set up across a network of Ethernet switches in contrast to conventional Ethernet paths where switches decide for themselves how to forward packets. One such proposal is described in WO 2005/099183. In WO 2005/099183 a network manager instructs each Ethernet switch along a route to store forwarding information. The switch uses the forwarding information to forward received data frames. A particular combination of identifiers in a data frame, such as a Virtual Local Area Network Identifier (VLAN ID or VID) and a destination address (DA) are used to differentiate traffic routes.
In a conventional destination-based forwarding scheme such as Ethernet, a single address entry in the bridging table at an intermediate switch is sufficient for all sources to reach a destination if all sources use the same route. In a network of the type described in WO 2005/099183 VIDs are used as route discriminators, allowing multiple routes to be followed to a destination. This requires a network management system to set up routes across the network, storing ‘state’ in each Ethernet switch. The state is an entry in the forwarding table at each switch along a chosen route. Assigning VIDs to different routes in a piecemeal fashion can quickly use a large number of different VIDs, requiring switches in a network to store a large amount of state. VIDs are limited in number, and excessive VID consumption is deprecated because it limits the number available of VIDs available for conventional bridged operation on the same infrastructure, if desired. Storing unnecessary fast-path state also incurs a direct cost penalty on the switches.
The present invention seeks to allocate identifiers in a more optimal manner.